Demonstration of reduced neoclassical energy transport in Wendelstein 7-X.

Research on magnetic confinement of high-temperature plasmas has the ultimate goal of harnessing nuclear fusion for the production of electricity. Although the tokamak1 is the leading toroidal magnetic-confinement concept, it is not without shortcomings and the fusion community has therefore also pursued alternative concepts such as the stellarator. Unlike axisymmetric tokamaks, stellarators possess a three-dimensional (3D) magnetic field geometry. The availability of this additional dimension opens up an extensive configuration space for computational optimization of both the field geometry itself and the current-carrying coils that produce it. Such an optimization was undertaken in designing Wendelstein 7-X (W7-X)2, a large helical-axis advanced stellarator (HELIAS), which began operation in 2015 at Greifswald, Germany. A major drawback of 3D magnetic field geometry, however, is that it introduces a strong temperature dependence into the stellarator's non-turbulent 'neoclassical' energy transport. Indeed, such energy losses will become prohibitive in high-temperature reactor plasmas unless a strong reduction of the geometrical factor associated with this transport can be achieved; such a reduction was therefore a principal goal of the design of W7-X. In spite of the modest heating power currently available, W7-X has already been able to achieve high-temperature plasma conditions during its 2017 and 2018 experimental campaigns, producing record values of the fusion triple product for such stellarator plasmas3,4. The triple product of plasma density, ion temperature and energy confinement time is used in fusion research as a figure of merit, as it must attain a certain threshold value before net-energy-producing operation of a reactor becomes possible1,5. Here we demonstrate that such record values provide evidence for reduced neoclassical energy transport in W7-X, as the plasma profiles that produced these results could not have been obtained in stellarators lacking a comparably high level of neoclassical optimization.

Femtosecond Transfer and Manipulation of Persistent Hot-Trion Coherence in a Single CdSe/ZnSe Quantum Dot.

Ultrafast transmission changes around the fundamental trion resonance are studied after exciting a p-shell exciton in a negatively charged II-VI quantum dot. The biexcitonic induced absorption reveals quantum beats between hot-trion states at 133 GHz. While interband dephasing is dominated by relaxation of the P-shell hole within 390 fs, trionic coherence remains stored in the spin system for 85 ps due to Pauli blocking of the triplet electron. The complex spectrotemporal evolution of transmission is explained analytically by solving the Maxwell-Liouville equations. Pump and probe polarizations provide full control over amplitude and phase of the quantum beats.

Impact of Magnetic Field Configuration on Heat Transport in Stellarators and Heliotrons.

We assess the magnetic field configuration in modern fusion devices by comparing experiments with the same heating power, between a stellarator and a heliotron. The key role of turbulence is evident in the optimized stellarator, while neoclassical processes largely determine the transport in the heliotron device. Gyrokinetic simulations elucidate the underlying mechanisms promoting stronger ion scale turbulence in the stellarator. Similar plasma performances in these experiments suggests that neoclassical and turbulent transport should both be optimized in next step reactor designs.

First Observation of a Stable Highly Dissipative Divertor Plasma Regime on the Wendelstein 7-X Stellarator.

For the first time, the optimized stellarator Wendelstein 7-X has operated with an island divertor. An operation regime in hydrogen was found in which the total plasma radiation approached the absorbed heating power without noticeable loss of stored energy. The divertor thermography recorded simultaneously a strong reduction of the heat load on all divertor targets, indicating almost complete power detachment. This operation regime was stably sustained over several energy confinement times until the preprogrammed end of the discharge. The plasma radiation is mainly due to oxygen and is located at the plasma edge. This plasma scenario is reproducible and robust at various heating powers, plasma densities, and gas fueling locations. These experimental results show that the island divertor concept actually works and displays good power dissipation potential, producing a promising exhaust concept for the stellarator reactor line.

Scaling of the Quantum Anomalous Hall Effect as an Indicator of Axion Electrodynamics.

We report on the scaling behavior of V-doped (Bi,Sb)_{2}Te_{3} samples in the quantum anomalous Hall regime for samples of various thickness. While previous quantum anomalous Hall measurements showed the same scaling as expected from a two-dimensional integer quantum Hall state, we observe a dimensional crossover to three spatial dimensions as a function of layer thickness. In the limit of a sufficiently thick layer, we find scaling behavior matching the flow diagram of two parallel conducting topological surface states of a three-dimensional topological insulator each featuring a fractional shift of 1/2e^{2}/h in the flow diagram Hall conductivity, while we recover the expected integer quantum Hall behavior for thinner layers. This constitutes the observation of a distinct type of quantum anomalous Hall effect, resulting from 1/2e^{2}/h Hall conductance quantization of three-dimensional topological insulator surface states, in an experiment which does not require decomposition of the signal to separate the contribution of two surfaces. This provides a possible experimental link between quantum Hall physics and axion electrodynamics.

Magneto-optics of massive dirac fermions in bulk Bi2Se3.

We report on magneto-optical studies of Bi2Se3, a representative member of the 3D topological insulator family. Its electronic states in bulk are shown to be well described by a simple Dirac-type Hamiltonian for massive particles with only two parameters: the fundamental band gap and the band velocity. In a magnetic field, this model implies a unique property-spin splitting equal to twice the cyclotron energy: Es=2Ec. This explains the extensive magnetotransport studies concluding a fortuitous degeneracy of the spin and orbital split Landau levels in this material. The Es=2Ec match differentiates the massive Dirac electrons in bulk Bi2Se3 from those in quantum electrodynamics, for which Es=Ec always holds.

Nanoscale morphology of multilayer PbTe/CdTe heterostructures and its effect on photoluminescence properties.

We study nanoscale morphology of PbTe/CdTe multilayer heterostuctures grown by molecular beam epitaxy on hybrid GaAs/CdTe (100) substrates. Nominally, the structures consist of 25 repetitions of subsequently deposited CdTe and PbTe layers with comparable thicknesses of 21 and 8 nm, respectively. However, the morphology of the resulting structures crucially depends on the growth temperature. The two-dimensional layered, superlattice-like character of the structures remains preserved only when grown at low substrate temperatures, such as 230 °C. The samples grown at the slightly elevated temperature of 270 °C undergo a morphological transformation to structures consisting of CdTe and PbTe pillars and columns oriented perpendicular to the substrate. Although the pillar-like objects are of various shapes and dimensions these structures exhibit exceptionally strong photoluminescence in the near infrared spectral region. At the higher growth temperature of 310 °C, PbTe and CdTe separate completely forming thick layers oriented longitudinally to the substrate plane. The observed topological transformations are driven by thermally activated atomic diffusion in the solid state phase. The solid state phase remains fully coherent during the processes. The observed topological transitions leading to the material separation in PbTe/CdTe system could be regarded as an analog of spinodal decomposition of an immiscible solid state solution and thus they can be qualitatively described by the Cahn-Hillard model as proposed by Groiss et al (2014 APL Mater. 2 012105).

Synthetic Mirnov diagnostic for the validation of experimental observations.

A synthetic Mirnov diagnostic has been developed to investigate the capabilities and limitations of an arrangement of Mirnov coils in terms of a mode analysis. Eight test cases have been developed, with different coil arrangements and magnetic field configurations. Three of those cases are experimental configurations of the stellarator Wendelstein 7-X. It is observed that, for a high triangularity of the flux surfaces, the arrangement of the coils plays a significant role in the exact determination of the poloidal mode number. For the mode analysis, torus and magnetic coordinates have been used. In most cases, the reconstruction of the poloidal mode number of a prescribed mode was found to be more accurate in magnetic coordinates. As an application, the signal of an Alfvén eigenmode, which has been calculated with a three-dimensional magnetohydrodynamics code, is compared to experimental observations at Wendelstein 7-X. For the chosen example, the calculated and measured mode spectra agree very well and additional information on the toroidal mode number and localization of the mode has been inferred.

Fabrication of magnetic artificial atoms.

We have fabricated gated vertical quantum dots made from a II-VI semiconductor heterostructure containing a paramagnetic quantum well. The absence of a known Schottky gate metal compatible with ZnSe based material precludes the traditional method of using a self-aligning shadow evaporated gate. Instead, we make use of a multi-step electron beam lithography process to surround a pillar with an insulating dielectric and gate. This process allows for the processing of dots with diameters down to 250 nm. Preliminary transport data confirming the magnetic nature of the resulting artificial atom are presented.

Sanitary inspection of wells using risk-of-contamination scoring indicates a high predictive ability for bacterial faecal pollution in the peri-urban tropical lowlands of Dar es Salaam, Tanzania.

Sanitary inspection of wells was performed according to World Health Organization (WHO) procedures using risk-of-contamination (ROC) scoring in the peri-urban tropical lowlands of Dar es Salaam, Tanzania. The ROC was assessed for its capacity to predict bacterial faecal pollution in the investigated well water. The analysis was based on a selection of wells representing environments with low to high presumptive faecal pollution risk and a multi-parametric data set of bacterial indicators, generating a comprehensive picture of the level and characteristics of faecal pollution (such as vegetative Escherichia coli cells, Clostridium perfringens spores and human-associated sorbitol fermenting Bifidobacteria). ROC scoring demonstrated a remarkable ability to predict bacterial faecal pollution levels in the investigated well water (e.g. 87% of E. coli concentration variations were predicted by ROC scoring). Physicochemical characteristics of the wells were not reflected by the ROC scores. Our results indicate that ROC scoring is a useful tool for supporting health-related well water management in urban and suburban areas of tropical, developing countries. The outcome of this study is discussed in the context of previously published results, and future directions are suggested.

Sources for constellation errors in modulated dispersion interferometers.

Dispersion interferometry (DI) is being employed on an increasing number of fusion experiments to measure the plasma density with a minimal sensitivity to vibrations. DIs employed in high-density experiments use phase modulation techniques up to several hundred kilohertz to enable quadrature detection and to be unaffected by variations of the signal amplitude. However, the evaluation of the temporal interferogram can be a significant source for phase errors and does not have an established processing method. There are two non-approximation-based methods currently in use: one using the ratio of amplitudes in the signal's Fourier spectrum and the other using its sectioned integration. Previously, the methods could not be used simultaneously since they differ in their respective calibration point. In this paper, we present a technique to use both phase evaluation methods simultaneously using quadrature correction methods. A comparison of their strengths and weaknesses is presented based on identical measurements indicating one to be more reliable in a more static measurement scenario, while the other excels in highly dynamic ones. Several comparative experiments are presented, which identify a significant error source in the phase measurement induced by polarization rotation. Since the same effect may be induced by Faraday rotation, the results may have direct consequence on the design of the ITER dispersion interferometer/polarimeter as well as the European DEMO's interferometer concept.

Fully electrical read-write device out of a ferromagnetic semiconductor.

We report the realization of a read-write device out of the ferromagnetic semiconductor (Ga,Mn)As as the first step to a fundamentally new information processing paradigm. Writing the magnetic state is achieved by current-induced switching and readout of the state is done by the means of the tunneling anisotropic magnetoresistance effect. This 1 bit demonstrator device can be used to design an electrically programmable memory and logic device.

Photoinduced Barkhausen effect in the ferromagnetic semiconductor (Ga,Mn)As.

Magnetization of ferromagnetic materials commonly occurs via random jumps of domain walls between pinning sites, a phenomenon known as the Barkhausen effect. Using strongly focused light pulses of appropriate power and duration we demonstrate the ability to selectively activate single jumps in the domain wall propagation in (Ga,Mn)As, manifesting itself as a discrete photoinduced domain wall creep as a function of illumination time. The propagation velocity can be increased over 7 orders of magnitude varying the illumination power density and the magnetic field.

Diffusion thermopower of (Ga,Mn)As/GaAs tunnel junctions.

We report the observation of tunneling anisotropic magnetothermopower, a voltage response to a temperature difference across an interface between a normal and a magnetic semiconductor. The resulting voltage is related to the energy derivative of the density of states in the magnetic material, and thus has a strongly anisotropic response to the direction of magnetization in the material. The effect will have relevance to the operation of semiconductor spintronic devices, and may indeed already play a role in correctly interpreting the details of some earlier spin injection studies.

Bayesian inference of spatially resolved Zeff profiles from line integrated bremsstrahlung spectra.

In nuclear fusion research, the effective ion charge Zeff, which characterizes the overall content of impurities, can be experimentally derived from the plasma electron-ion bremsstrahlung, given the electron density ne and temperature Te. At Wendelstein 7-X, a multichannel near-infrared spectrometer is installed to collect the plasma bremsstrahlung along 27 lines of sight covering more than half the plasma cross section, which provides information on Zeff over the entire plasma radius. To infer spatially resolved Zeff profiles, a Bayesian model is developed in the Minerva framework. Zeff, ne, and Te profiles are modeled as Gaussian processes, whose smoothness is determined by hyperparameters. These profiles are transformed to fields in Cartesian coordinates, given the poloidal magnetic flux surfaces calculated by the variational moments equilibrium code. Given all these physical quantities, the model predicts line-of-sight integrals of near-infrared bremsstrahlung spectra. The model includes the predictive (forward) models of the interferometer, Thomson scattering system, and visible and near-infrared spectrometers. Given the observations of all these diagnostics, the posterior probability distribution of Zeff profiles is calculated and shown as an inference solution. The smoothness (gradient) of the profiles is optimally chosen by Bayesian Occam's razor. Furthermore, wall reflections can significantly pollute the measurements of the plasma bremsstrahlung, which leads to over-estimation of Zeff values in the edge region. In the first results presented in this work, this problem does not appear, and the posterior samples of Zeff profiles are overall plausible and consistent with Zeff values inferred, given the data from the single-channel visible spectrometer.

Separation of stages in the development of the "T" cells involved in cell-mediated immunity.

Density distribution analysis in continuous gradients of albumin has been used to study the development of cytotoxic lymphocytes (CL), to separate and characterize the progenitors of CL, and to determine their relationship to subpopulations of T cells. CL progenitors in the thymus were a homogeneous, medium-density population, distinct from the typical, dense, thymus small-lymphocyte. Activity seemed to be associated with one minor subpopulation of cells with surface antigenic properties characteristic of peripheral T cells (high levels of H-2 antigen, low levels of theta-antigen). CL progenitors in the spleen differed from those in the thymus and normally had the high buoyant density of typical small T lymphocytes. In states of antigenic stimulation, some lighter-density CL progenitors were found in the spleen. The buoyant density of the CL population developing in the spleens of immunized animals showed progressive changes with time. Early, "immature" CL had the light-density characteristics of large, activated lymphocytes. As the response developed, the density of the CL population increased, and finally approached that of CL progenitors and of typical small lymphocytes. The data suggest that density subpopulations of T cells represent stages in the development of immunocompetent cells. Possible differentiation pathways of T lymphocytes in the thymus and in the spleen are discussed.

Antigenic specificity of the cytolytic T lymphocyte response to murine sarcoma virus-induced tumors. III. Characterization of cytolytic T lymphocyte clones specific for Moloney leukemia virus-associated cell surface antigens.

Cytolytic T lymphocyte (CTL) clones specific for Moloney leukemia virus (MoLV)-derived tumor cells were generated by placing limiting numbers of C57BL/6 responder cells into mixed leukocyte-tumor cell microcultures. Under appropriate conditions (presence of stimulating tumor cells, accessory cells, and T cell growth factor), such cloned CTL could readily be expanded to provide large numbers of homogeneous, highly cytolytic CTL populations for further characterization. Using four target-cell types, three specificity patterns were observed: one reactive with the syngeneic MoLV-derived tumor only, one cross-reactive with an allogeneic MoLV-derived tumor, and one cross-reactive with normal allogeneic cells. Subclones derived from these three types of clones exhibited a high degree of stability in terms of lytic activity and specificity over a 4-mo period of observation. Three clones analyzed by flow cytofluorometry using monoclonal antibodies were all found to be of the Lyt-1+2+ phenotype. Furthermore, lysis of target cells by all of six clones tested was inhibited by anti-H-2Db (but not by anti-H-2Kb) monoclonal antibodies, demonstrating H-2Db-restriction at the clonal level.

Generation of cytotoxic T lymphocytes in vitro. IV. Functional activation of memory cells in the absence of DNA synthesis.

Re-exposure of day 14 mixed leukocyte culture (MLC) cells to the original stimulating alloantigens (secondary response) has previously been shown to result in significant proliferation and in rapid reappearance of high levels of cytolytic T-lymphocyte (CTL) activity within the next 4 days. Moreover, evidence has been presented that CTL precursor cells in day 14 MLC populations, while they derived from cells were large at peak of the primary response (day 4) were themselves small lymphocytes which developed into large CTL after restimulation. In this study, inhibition of DNA synthesis by cytosine arabinoside (ARA-C) was used to investigate whether CTL formation could be dissociated from proliferation during the secondary response. It was found that within the first 24 h after restimulation (a) CTL activity increased 6-to-20-fold, (b) 60-70% of the small T lymphocytes became medium- to large-sized cells, and (c) both events were independent of DNA synthesis. By using two successive cell separations by velocity sedimentation at unit gravity, before and after stimulation of day 14 MLC cells for 24 h in the presence or absence of ARA-C, direct evidence was obtained that small CTL precursor cells developed into large CTL, irrespective of DNA synthesis. The presence of ARA-C for periods longer than 24 h inhibited any further increase in CTL activity, in contrast to a parallel increase in lytic activity and cell number from day 1 to day 4 in control restimulated cultures. Taken together with the finding that 90% of the medium- and large-sized lymphoid cells in control restimulated cultures underwent DNA synthesis within 24 h, these results thus suggest that during a secondary MLC response there is initially a differentiation step leading to the formation of CTL which, although it can be clearly dissociated from DNA synthesis, is under normal conditions followed by proliferation of these effector cells.

Quantitation and clonal isolation of cytolytic T lymphocyte precursors selectively infiltrating murine sarcoma virus-induced tumors.

A limiting dilution mixed leukocyte-tumor cell microculture system was used to quantitate cytolytic T lymphocytes and their precursors (CTL-P), which infiltrate tumors induced by injection of Moloney sarcoma-leukemia virus (MSV-MoLV) complex into C57BL/6 mice. Leukocyte populations obtained from tumors collected on day 10 after virus injection were found to contain significantly higher frequencies of operationally defined (tumor-specific) CTL-P than either peripheral blood leukocytes (PBL) or spleen cells from the same animals. When these frequencies were normalized according to the content of Lyt-2+ T cells in each tissue, average CTL-P frequencies were found to be 1/9 in tumor-infiltrating cells vs. 1/41 in PBL. These results directly demonstrate selective accumulation of CTL-P in the tumor mass. A number of clonal isolates obtained from tumor-infiltrating leukocyte populations were expanded and studied for cytolytic activity and specificity. Of 11 isolates, 10 were found to have high cytolytic activity, leading to 50% lysis of the syngeneic MoLV-derived tumor target cells in 3.5 h at lymphocyte:target cell ratios ranging from 0.5:1 to 3.2:1. Furthermore, five randomly selected clones showed H-2 restriction by their selective lytic activity against MoLV-derived syngeneic MBL-2 target cells and their lack of activity against either MoLV-derived allogeneic (LSTRA) tumor cells or against syngeneic (EL4) or allogeneic (P815) target cells unrelated to MoLV.